Antenna



Dec. 29, 1953 w. SICHAK 2,664,508

ANTENNA Filed July 9, 1945 FIG.|

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2O 25 23 F A5 24 l8 [9 f k W F162 |4 l l n ALTFFUDE FIGS INWWKR WILLIAM SICHAK BY RANGE ATTORNEY Patented Dec. 29, 1953 ANTENNA William Sichak, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application July 9, 1945, Serial No. 604,029 8 Claims. (01. 250-4315) This invention relates to antennas for radio communication systems and particularly to antennas having means for obtaining an energy distribution pattern closely approximating the relation of C8020 in elevation, where a is the radiation angle measured from the axis of the radiating means, or of the undistorted or symmetrical beam pattern, i. e., a pattern normally produced by an undistorted or conventionally shaped radiator or reflector.

' In certain radio object-locating systems such as used for blind bombing from airplanes, it is desirable that radiation of energy from an antenna have (1) an elevation pattern that produces a constant echo (within about 2 or 3 db) from an isotropic target, independent of its range, (2) an azimuth beam Width that is constant at all useful elevation angles, (3) as high a gain as possible consistent with (1), and (4) a well defined polarization. Thus it is desirable that the energy distribution in a vertical plane be such that the variation of energy density versus the .radiation angle approximates a cosecant-squared function. This distribution provides generally uniform echo strength for targets disposed at substantially equal elevation with respect to the antenna regardless of Whether the target is close in or distant.

Prior airborne antennas used for providing a c30 pattern for search purposes have presented certain difficulties with extensive altitude variation. This results from the fact that at different altitudes for the same approximate range the antenna must be tilted to follow a target, and such tilting detracts from the desired uniform pattern of ground return. While modifications of paraboloidal antennas have been proposed in attempts to overcome the disadvantages and to provide an antenna having the above itemized characteristics, paraboloidal antennas encounter certain difliculties when coverage is extended out to large angles (about 70), and when the aperture becomes long compared to its heighth.

It is an object of the present invention to provide an antenna adapted to produce a radiation pattern of c 0 configuration which has a uniform ground return characteristic over a reasonable range of altitudes.

It is another object of the invention to provide an antenna adapted to vary the radiation pattern so that, at different altitudes, the intensity of the beam is uniform over the area of the ground plane While maintaining an approximation to a csc o configuration in elevation.

Other novel features and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawingsin which:

Fig. 1 is a side elevation view of one embodiment of the antenna according to the present invention; v

Fig. 2 is a diagrammatic view of the antenna according to the invention illustrating the angular tilts of the radiating means and the radiation patterns produced;

Fig. 3 is a diagram illustrating the radiation patterns produced for different altitudes.

One embodiment of the present invention contemplates an antenna having a metallic reflector illuminated by a so-called pillbox type feed or radiating source. The characteristics of a pillbox type feed cooperate with the characteristics of the reflector used to provide the desired c.90 0 radiation pattern. The word pillbox as used herein, contemplates a structure comprising a parabolic cylindrical reflecting surfac mounted between plane parallel metallic Walls or plates: which serve to enclose the ends of the parabolic cylinder. The spacing between the plates and consequently the axial dimension of th cylinder is preferably less than half the wavelength of the feed energy. However, it is to be understood that the invention is not limited to the use of the pillbox type feed, as it is contemplated to use any suitable source for illuminating the antenna reflector. For example, linear array type radiating elements such as those described in copending applications of Luis W. Alvarez, Serial No. 542,287, filed June 27, 1944, now Patent No. 2,480,208, issued August 30, 1949, and Serial No. 509,790, filed November 10, 1943, now Patent No. 2,605,413, issued July 29, 1952, and of Jack Steinberger. Serial No. 584,227, filed March 22, 1945, now abandoned, respectively, may be utilized with the present invention.

whereby an unsymmetrical radiation pattern,

which generally varies as the square of the cosecant of the angle measured from the radiation axis, is produced. Reflector I0 is located so that its focal line normally lies in a substantially horizontal plane. The non-parabolic portion may be located in the upper or lower portion of the reflector H1 depending on whether it is desired to direct the resultant beam pattern downwardly or upwardly, respectively. The reflector l0 may, of course, take any shape that will. produce a cosecant squared radiation pattern, and the term cosecant squared reflector will be used herein to designate and include any such reflector.

Reflector I0 is mounted on any suitable support; for example, in an airborne assembly, on a spinner H of any suitable design which may be totatably supported from the under portion of the airplane. One means for supporting reflector I0 is shown in Fig. 1 in which bars or similar rigid members 12 have one of their ends secured to the upper edge portions of reflector Ill and their opposite ends securely attached by any suitable means to spinner H.

' Also supported by spinner II is the radiating element 14 which is" shown as a pillbox type feed, but which may be a linear array as referred to hereinbefore. Pillbox i i comprises parallel plate members it: and I6 closing the ends of a parabolic reflecting cylinder 17, and affording a rectangular opening or mouth Iii. Mouth 18 may be flared slightly, as shown,if desired. Pillbox M is located so that it will illuminate reflector l0.

Pillbox i4 is supported in any suitable manner so that it may pivot about an axis which is coincident with the focal line F of the reflector. For example, outwardly extending projections or lugs 19 may be mounted on parabolic cylinder [1 adjacent to and on opposite sides ofmouth 18 with the axis of lugs is preferably being: coincident with the focal line F. Strut or bracket members as have one of their ends securely mounted on spinner H and their opposite ends pivotally attached to the extending ends of lugs it. If desired, reflector it may receive its entire, or additional, support by meansof strut members 21 and 22 connected to the upper and lower edge portions at its extremities and to the lugs 19. Pillb'ox U3 is excited. in any conventional manner as by a waveguide 23 entering the mouth [-8.

Means are provided for controlling the tilt or "disposition of pillbox i4 relative to reflector is. One means of accomplishing this manually is shown in Fig. 1 by way of example. A memher as is rigidly mounted on the outside surface of plate i5, spaced to the rear of the axis of rotation of'pillbox id (away from reflector ill) and preferably near the vertex region" of the parabolic cylinder IT. A rod 2'5 has one of its ends associated with member 2 3, as shown. 'I'heinterme'di ate portion of rod 25 may be threaded for en'- gagement with a'threaded aperture "in spinner ii as at 26, and a handle 2? maybe provided'onthe opposite end of rod 25. By "turning handle 21 (a motor, not shown, may be provided for this purpose) pillbox lid may be tilted about its pivotal axis any desired amount. thereby varying the angular relation of the beam of radiant energy emanating from thepillbox toward reflector I 0. Thus the tilt of pillbox i4 may 'beadjusted to provide a radiation pattern wherein groundreturn. will be generally uniform over a range of altitudes, as more fully described hereinafter. Fig. 2 illus'tratesin diagrammatic form the cylindrical reflector i ll with the radiating element inathe form of pillbox 4 4 constructed and arranged as hereinbeforedescribed with reference to Fig.1. Figs. 2 ands-alsozshow csc B beam patterns pro- 4 duced with different tilts of the radiator M. The two patterns, representing tilts used at two different altitudes, are designated by solid and broken curved lines respectively. Fig. 2 is a rough approximation showing how the changing of the tilt angle of the pillbox feed l4 varies the reflected beam pattern so that a plane representing a cross-sectional area of this beam of equal power intensity would be parallel to the ground plane, with difierent altitudes. Another way of describing this is illustrated with reference to Fig. 3 where instead of tilting the generally triangularly shaped pattern which forms the silhouetteof the beam pattern, the pattern itself is altered by changing the angle or radiation from the antenna, whichangle represents the 0 of the csc e pattern. The solid and broken straight lines of Fig. 3 indicate the approximate triangulation pattern of the respective beams at different altitudes to the point 00 (the maximum slant range) For the present description it assumed that the antenna is airborne and the reflector I0 is adapted to direct a beam in a downward direction. It will be understod that with a pillbox or other linear array type of radiating source, the reflected beam will be substantially a so-called straight line berm, that is, the pattern at any one instant will define a straight line, or When the'azimuth width and range length of the beam are considered, a rectangular trace, in the ground plane. As is well known, the azimuth beam width of the ground plane pattern depends on the length of the array, the azimuth dimension of the beam at the reflector, the scanning angle, and the rltitude of the antenna above ground. Similarly the length n of the beam .(Fig. 2) is dependent on the width of the mouth 18 between plates l5 and iii, the .directivity of the antenna in elevation, and the altitude.

It'will be apparent that with a fixed relationship between reflector l0 and radiating source in as in antennss heretofore used the dimensions of the beam pattern in the ground plane will increase'with an increase in altitude, with a resultant decrease in gain.

According to the present invention the pattern itself is varied to obtain a substantially consttnt azimuth beam width and constant length n in the ground plane at diiferent altitudes so that the intensity of the beam is uniform over the area in theground plane. This is accomplished by slightly tilting or rotating the radiating element or pillbox Hi about the focal line F so that the primary beam from pillbox i4 is directed toward reflector ii) at different angles. This primary beam accordingly impinges on difieren-t portions of the reflecting surface of reflector 19. Due to the shape of this reflecting surface, different patterns of the secondary beam reflected by reflee-tor i5 will be obtained in which the length n ii the ground plane the intensity of the beam is maii'itained substantially constant.

It will be apparent that the antenna according to the presentinvention is capable of producing a pattern in elevation that obtains a substantially constant ground return or echo with high gainirom a target independent of its range or the altitude of the antenna. Also that the azimuth beam width is maintained substantially =.constan't at all useful elevation angles.

While the present description is particularly directed to the-radiating means being tilted rela- 'tive to a'flxed re'flecter, it will 'be understood that substantially the same results may be obtained by tilting the reflector relative to a fixed radiator.

While there has been described preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention.

What is claimed is:

1. A directive antenna comprising a source of directional radio wave energy, reflector means to reflect said energy in a pattern that varies as the square of the cosecant of an angle that is measured from the axis of the reflector means, and means for shifting said pattern with respect to said axis, said last named means comprising means for adjusting the angular positions of said source and said reflector with respect to each other.

2. An antenna to illuminate with equal intensity a plurality of targets disposed on a surface, comprising a directional radiator and a reflector spaced from said surface and positionable along a line that is normal to and bisects one of the sides of said surface, said reflector comprising means to obtain a radiation pattern that varies as the sduare of the cosecant or the angle measured from an axis of said reflector, means for tilting said radiator with respect to said reflector, the angle of tilt being a function of the position of said reflector along said line, whereby the illumination on said targets may be maintained substantially constant for each position of said antenna along said line.

3. The device according to claim 2, wherein said surface comprises an elongated surface and said antenna is positionable along a line that is normal to and bisects one of the short sides of said elongated surface.

4. In combination, a source of directional radiant energy a reflector adapted to be illuminated by said source to produce a directional radiant energy pattern that varies as the square of the cosecant or an angle measured from the axis of said reflector comprising means to reflect a portion of said energy in a direction that is substantially parallel to said axis, means to reflect the remaining portions of said energy in a direction that is non-parallel to said axis, and means to shift said portions with respect to said axis, said last named means including means for adjusting the angular positions of said source and said reflector with respect to each other.

5. The device according to claim 4, wherein said shifting means comprises a directional radiator tiltably positioned at the approximate focus of said reflector.

6. An antenna for an airborne radio objectlocating system, comprising a cylindrical reflector having a curvature according to geometric optics to produce an energy distribution pattern which varies substantially as the square of the cosecant of the angle measured from the radiation axis of said antenna, a pillbox type radiating element adapted to radiate electromagnetic energy in an incident beam, narrow in elevation, toward said reflector, said pillbox comprising a parabolic cylindrical reflecting surface having a short focal line and a pair of parallel plate members enclosing the ends of said surface and forming therewith a rectangular aperture having an axis perpendicular to said focal line, said pillbox being disposed with its axis substantially coincident with a focal line of said reflector, said pillbox being mounted to pivot about its axis relative to said reflector, and means for tilting said pillbox about its axis to change the angle between said incident beam and said reflector, whereby said distribution pattern may be varied to maintain substantially uniform radiation intensity in the ground plane for different altitudes of said antenna.

7. An airborne antenna, comprising a directional radiatin element adapted to radiate electromagnetic energy, a reflector adapted to be illuminated by said element, said reflector having a cosecant squared 6 distribution pattern producing curvature, where 0 is the angle measured from the radiation axis of said antenna, said reflector being located so that its focus is at the apparent center of radiation of said radiating element, and means for tilting said radiating element through an angle about a horizontal axis through said center of radiation thereby varying the angle of the energy beam impinging on said reflector, whereby different beam patterns may be produced at different altitudes of the antenna to provide a uniform ground return at said difierent altitudes.

8. An antenna for an airborne radio objectlocating system, comprising a directional radiating element adapted to radiate waves of electromagnetic energy, a substantially cylindrical reflector having a radiant energy distribution pattern which varies substantially as the square of the cosecant of the angle measured from the radiation axis of said antenna, said reflector being illuminated by said element, means for tiltin said element and said reflector relative to each other through an angle in a plane perpendicular to the focal line of said cylindrical reflector, whereby the energy incident to said reflector impinges on different portions of the reflecting surface thereof, and means for controlling said angle of tilt to produce substantially uniform ground return for different angular dispositions of said radiant energy distribution pattern.

WILLIAM SICHAK.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,625,946 Laird Apr. 26, 1927 1,932,469 Lieb et a1 Oct. 31, 1933 2,083,242 Runge June 8, 1937 2,156,653 Ilberg May 2, 1939 2,283,935 King May 26, 1942 2,429,601 Biskeborn et al Oct. 28, 1947 2,434,253 Beck June 13, 1948 2,555,123 Gardner May 29, 1951 2,575,058 King Nov. 13, 1951 

